This invention relates to a method for operating pulverized solid fuel-fired furnaces and to a fuel and air compartment arrangement for such furnaces operable in accordance with the method which is applicable to a wide range of solid fuels and which when employed with a pulverized solid fuel-fired furnace is capable of providing a favorable emissions control operation.
Pulverized solid fuel has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The tangential firing technique involves introducing the pulverized solid fuel and air into a furnace from the four corners thereof so that the pulverized solid fuel and air are directed tangentially to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the pulverized solid fuel and the air, stable flame conditions, and long residence time of the combustion gases in the furnaces.
Recently though, more and more emphasis has been placed on the minimization as much as possible of air pollution. In this connection, with reference in particular to the matter of NO.sub.x control it is known that oxides of nitrogen are created during fossil fuel combustion primarily by two separate mechanisms which have been identified to be thermal NO.sub.x and fuel NO.sub.x. Thermal NO.sub.x results from the thermal fixation of molecular nitrogen and oxygen in the combustion air. The rate of formation of thermal NO.sub.x is extremely sensitive to local flame temperature and somewhat less so to local concentration of oxygen. Virtually all thermal NO.sub.x is formed at the region of the flame which is at the highest temperature. The thermal NO.sub.x concentration is subsequently "frozen" at the level prevailing in the high temperature region by the thermal quenching of the combustion gases. The flue gas thermal NO.sub.x concentrations are, therefore, between the equilibrium level characteristic of the peak flame temperature and the equilibrium level at the flue gas temperature.
On the other hand, fuel NO.sub.x derives from the oxidation of organically bound nitrogen in certain fossil fuels such as coal and heavy oil. The formation rate of fuel NO.sub.x is strongly affected by the rate of mixing of the fossil fuel and air stream in general, and by the local oxygen concentration in particular. However, the flue gas NO.sub.x concentration due to fuel nitrogen is typically only a fraction, e.g., 20 to 60 percent, of the level which would result from complete oxidation of all nitrogen in the fossil fuel. From the preceding it should thus now be readily apparent that overall NO.sub.x formation is a function both of local oxygen levels and of peak flame temperatures.
Over the years, there have been numerous modifications made to the standard tangential firing technique. Many of these modifications, and in particular those that have been suggested most recently, have been proposed primarily in the interest of achieving an even better reduction of emissions through the use thereof. The resultant of one such modification is the firing system that forms the subject matter of U.S. Pat. No. 5,020,454 entitled "Clustered Concentric Tangential Firing System", which issued on Jun. 4, 1991 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,020,454, there is provided a clustered concentric tangential firing system that is particularly suited for use in fossil fuel-fired furnaces. The clustered concentric tangential firing system includes a windbox. A first cluster of fuel nozzles are mounted in the windbox and are operative for injecting clustered fuel into the furnace so as to thereby create a first fuel-rich zone therewithin. A second cluster of fuel nozzles are mounted in the windbox and are operative for injecting clustered fuel into the furnace so as to thereby create a second fuel-rich zone therewithin. An offset air nozzle is mounted in the windbox and is operative for injecting offset air into the furnace such that the offset air is directed away from the clustered fuel injected into the furnace and towards the walls of the furnace. A close coupled overfire air nozzle is mounted in the windbox and is operative for injecting close coupled overfire air into the furnace. A separated overfire air nozzle is mounted within the burner region of the furnace so as to be spaced from the close coupled overfire air nozzle and so as to be substantially aligned with the longitudinal axis of the windbox. The separated overfire air nozzle is operative for injecting separated overfire air into the furnace.
The result of another such modification is the firing system that forms the subject matter of U.S. Pat. No. 5,146,858, which is entitled "Boiler Furnace Combustion System", and which issued on Sep. 15, 1992. In accordance with the teachings of U.S. Pat. No. 5,146,858, a boiler furnace combustion system is provided of the type that typically includes main burners disposed on side walls of or at corners of a square-barrel-shaped boiler furnace having a vertical axis with the burner axes being directed tangentially to an imaginary cylindrical surface coaxial to the furnace. Moreover, in this type of boiler furnace combustion system,air nozzles are disposed in the boiler furnace at a level above the main burners so that unburnt fuel left in a reducing atmosphere or a lower oxygen concentration atmosphere of a main burner combustion region can be perfectly burnt by additional air blown through the air nozzles. The boiler furnace combustion system, as taught in U.S. Pat. No. 5,146,858, is particularly characterized in that two groups of air nozzles are disposed at higher and lower levels, respectively. More specifically, the air nozzles at the lower level are provided at the corners of the boiler furnace with their axes directed tangentially to a second imaginary coaxial cylindrical surface having a larger diameter than the first imaginary coaxial cylindrical surface. The air nozzles at the higher level, on the other hand, are provided at the centers of the side wall surfaces of the boiler furnace with their axes directed tangentially to a third imaginary coaxial cylindrical surface having a smaller diameter than the second imaginary coaxial cylindrical surface.
The result of yet another such modification is the firing system that forms the subject matter of U.S. Pat. No. 5,195,450 entitled "Advanced Overfire Air System for NO.sub.x Control", which issued on Mar. 23, 1993 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,195,450, there is provided an advanced overfire air system for NO.sub.x control, which is designed for use in a firing system of the type that is particularly suited for use in fossil fuel-fired furnaces. The advanced overfire air system for NO.sub.x control includes multi-elevations of overfire air compartments consisting of a plurality of close coupled overfire air compartments and a plurality of separated overfire air compartments. The close coupled overfire air compartments are supported at a first elevation in the furnace and the separated overfire air compartments are supported at a second elevation in the furnace so as to be spaced from but aligned with the close coupled overfire air compartments. Overfire air is supplied to both the close coupled overfire air compartments and the separated overfire air compartments such that there is a predetermined most favorable distribution of overfire air therebetween, such that the overfire air exiting from the separated overfire air compartments establishes a horizontal "spray" or "fan" distribution of overfire air over the plan area of the furnace, and such that the overfire air exits from the separated overfire air compartments at velocities significantly higher than the velocities employed heretofore.
The flames produced at each pulverized solid fuel nozzle are stabilized through global heat-and mass-transfer processes. A single rotating flame envelope ("fireball"), centrally located in the furnace, provides gradual but thorough and uniform pulverized solid fuel-air mixing throughout the entire furnace.
The efforts to control NO.sub.x emissions such as exemplarily noted above have also been paralleled by efforts to improve the wastage or corrosion of the sidewalls of a furnace operating in the stoichiometric regimes often associated with low NO.sub.x operation. Reducing conditions along the sidewalls initiate or accelerate wastage of these areas of the furnace.
Thus, although firing systems constructed in accordance with the teachings of the three issued U.S. patents to which reference has been made hereinbefore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such firing systems to be improved. More specifically, a need has been evidenced in the prior art for a new and improved tangential firing system that would enable more flexibility in the control of undesirable emissions such as nitric oxides. Additionally, a need has been evidenced in the prior art for a new and improved tangential firing system which would enhance the resistance to corrosion along the waterwalls of the furnace.
To thus summarize, a need has been evidenced in the prior art for a new and improved tangential firing system that when employed with a pulverized solid fuel-fired furnace is capable of optimally reducing undesirable emissions.